1. Field of the Invention
The present invention relates to a MOSFET (Metal Oxide Semiconductor Field Effect Transistor) including a current mirror FET for a current detection therein, and more particularly to the protection of a gate insulating film of a current mirror FET in a MOSFET.
2. Description of the Background Art
In FIG. 1, there is shown a conventional MOSFET including a current mirror FET for a current detection therein.
In the drawing, there are two MOSFETs, a power MOSFET 1 and a current mirror MOSFET 2. Their drains D.sub.P and D.sub.M are connected in common to a drain terminal 3, and their gate electrodes G.sub.P and G.sub.M are coupled in common to a gate terminal 4. Their sources S.sub.P and S.sub.M are connected to different source terminals 5 and 6, respectively. In this case, a drain current I.sub.D is obtained by the following formula: ##EQU1## wherein W: channel width; L: channel length; un: surface mobility of electron; C.sub.O X : capacitance by oxide film; V.sub.G : voltage applied to gate electrode; V.sub.T : threshold voltage; and V.sub.D : drain voltage.
As apparent from formula (1), by varying the channel width, the drain current I.sub.D is changed. When the channel width of the current mirror MOSFET 2 is determined to one several thousandth to one several tens of thousandth as compared with that of the power MOSFET 1, the current I.sub.M flowing in the current mirror MOSFET 2 becomes one several thousandth to one several tens of thousandth of the current I.sub.P flowing in the power MOSFET 1.
In FIG. 2, there is shown a conventional circuit for detecting a current flowing in the power MOSFET 1 by using the current mirror MOSFET 2. In this method, the detection can be carried out in accordance with the same principle as that of a shunt of a tester or the like.
In FIG. 2, the source terminal 5 of the power MOSFET 1 is directly connected to one terminal of a load 7, and the source terminal 6 of the current mirror MOSFET 2 is also linked to the one terminal of the load 7 via a resistor 8 having a small resistance R interposed in series therebetween. The other terminal of the load 7 is connected to the ground. Now, the voltage between both the ends of the resistor 8 can be obtained in the following formula: EQU V=R.multidot.I.sub.M ( 2)
The total current I flowing in the load 7 results in the following formula: EQU I=I.sub.M +I.sub.P ( 3)
The current I.sub.P flowing in the power MOSFET 1 is obtained in the following formula: ##EQU2## wherein W.sub.P and W.sub.M are the channel widths of the power MOSFET 1 and the current mirror MOSFET 2, respectively. Accordingly, the total current I is obtained in the following formula: ##EQU3## Therefore, the total current I of the load 7 can be obtained from the voltage V between the ends of the resistor 8.
In the above described conventional MOSFET including the current mirror FET therein, when W.sub.P &gt;&gt;W.sub.M and the gate electrode of the current mirror MOSFET 2 is small, its gate capacity is very small such as several pF, and its source S.sub.M is directly linked to the source terminal 6. Hence, when an operator touches the source terminal 6 by his hand, static electricity charged on the hand or the like is directly given to the gate oxide film via the source terminal 6, and thus a very high voltage V=Q/C is applied to the gate oxide film to cause a dielectric breakdown therein. In turn, the gate capacity of the power MOSFET 1 is large, and it is far durable to the breakdown due to the static electricity as compared with the current mirror MOSFET 2.